The dialog is bit too big and that button is lengthy because of the design, not to mention the wasted dialog space over all. From the workability aspect, it's not a big deal, but from the design standpoint, it can be improved so it looks nice proportion-wise.

The "Show Details" button opens up a list so that you can see what goes on during the installation and update process. It has a lot of room at the top because of the amount of information that it has the potential to store. With a lot of information in the dialog, this was the most aesthetic choice. Plus the Finish button is hidden.

Why not use /dialog -s and resizes the dialog as and when you need to?

cause then i would have to move buttons around, and without actually editing the script...i haven't found a way to do that yet :P